Lebanon says three former security chiefs and the commander of the country's Presidential Guards are suspects in the U.N. probe into the assassination of former Prime Minister Rafik Hariri.

Lebanese Prime Minister Fouad Siniora confirmed that police detained the three security chiefs earlier Tuesday, and that the Guards commander had been summoned to appear before U.N. investigators.

The Guards commander, Brigadier General Mustafa Hamdan, along with the men detained - Jamil al-Sayed, the former head of the country's General Security; Raymond Azar, the ex-chief of military intelligence; and, Ali Hajj, the former police chief - are all considered to be staunchly pro-Syrian.

Many in Lebanon blamed Syria for the February 14 bombing in Beirut that killed Mr. Hariri and 20 other people. Damascus denied any involvement.

The killing sparked massive anti-Syrian protests and increased international pressure on Syria to withdraw its troops from Lebanon. Damascus ended its 29-year military presence in the country in April.